Layers of Plywood

Discussion in 'Wooden Boat Building and Restoration' started by rasorinc, Jan 10, 2014.

  1. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    With my bottom boat dimensions being 8' in width and plans calling for two layers of 3/8 plywood and bottom batts spaced 9" apart I can run one layer side to side and the second layer front to rear. Any benifit or drawback to turning one layer 90 degrees to the other? Bottom will be fiberglassed over. Thanks
     
  2. gonzo
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    gonzo Senior Member

    It won't make any difference, except maybe allow you to cut the plywood with less waste. The layers are already at 90 degrees.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Yes, there could be an issue, as plywood is considerably weaker in it's cross grain orientation. It depends on the plans. I have a skiff plan with a double planked bottom. To maintain the stiffness and strength this provides (two 1/2" layers), I've carefully offset the seams on each layer, so fore and aft grain orientation can be insured, yet the seams well overlapped.

    Additionally, you can gain a lot more stiffness with the longitudinal orientation (eliminating some stiffener weight), if you cant each layer at 10 to 12 degrees to each other. This can be wasteful to a degree, but it all depends on what you want as a designer.
     
  4. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    I'd second PAR but add that plywood can also have different veneer thicknesses in its make up. This affects the strength cross grain. One other answer if you have a good veneer merchant is to make your own ply, then you can put diagonal cores inside the layup. You don't need a press, but a vac bag is ideal, however just sandwiching between heavy sheets of MDF/faced Chip board is good enough if weighted solidly.

    The key to making good ply is the adhesive and spreading it properly to keep the weight down ie do not over saturate. It is not hard to do if you know how to butt joint veneers properly. Don't forget any scarf joints should be 1 in 7 or greater for maximum joint strength. Again not hard to do, more of a clamping issue if very wide.

    Good marine ply is hard to find and not cheap. Make sure if you bond the two 3/8" (9mm) sheets together that they are glued properly. I've seen a lot of two sheets of 5 or 6mm stuff supposedly stuck together with huge voids.
     
  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The batten spacing is 9" and the hull is fiberglassed. I don't think it can make much of a difference. Structurally, it will be small squares.
     

  6. rasorinc
    Joined: Nov 2007
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    Location: OREGON

    rasorinc Senior Member

    thanks everyone for your input. I reallly appreciate all the comments.
     
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